class link budget

3 June 2008

Satellite Satellite

Communication Communication

Link BudgetLink Budget

3 June 2008

Link budget is actually the sum of all the losses between: Transmitter - Satellite & Link budget is actually the sum of all the losses between: Transmitter - Satellite & back down to a Receiver. back down to a Receiver. These losses are reduced by any gain you have at the transmitter, satellite or These losses are reduced by any gain you have at the transmitter, satellite or receiver. So in order to see if your signal is still going to be big enough to use after it receiver. So in order to see if your signal is still going to be big enough to use after it has been sent to a receiver via satellite, the gains and losses are effectively added has been sent to a receiver via satellite, the gains and losses are effectively added together and the result will be the net gain or loss. A loss means your signal has got together and the result will be the net gain or loss. A loss means your signal has got smaller, and a gain means it has got bigger. smaller, and a gain means it has got bigger.

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Satellite Link DesignSatellite Link Design

The four factors related to satellite system design:– The weight of satellite –Heavier the satellite higher the cost

And overall dimensions should be small since the space craft must fit into the launch vehicle. The weight of the satellite depends on various factors such as the number and the output power of the transponder and the weight of station keeping fuel. And as power requirements increases dimension of solar cell increases and the weight again increases.

– The choice frequency band – Atmospheric propagation effects

Rain in the atmosphere attenuate the radio signal, This effect is more sever at higher frequencies.

– Multiple access technique


• The major frequency bands are 6/4 GHz, 14/11 GHz and 30/20 GHz (Uplink/Downlink)

• At geostationary orbit there is already satellites using both 6/4 and 14/11 GHz every 2 (minimum space to avoid interference from uplink earth stations) -> Additional satellites higher BW

• Low earth orbit (LEO) & medium earth orbit (MEO) satellite systems are closer and produces stronger signals but earth terminals need omni directional antennas

3 June 2008


The design of any satellite communication is based on – Meeting of minimum C/N ratio for a specific

percentage of time. – Carrying the maximum revenue earning traffic at

minimum cost.

Link BudgetsLink Budgets

C/N ratio calculation is simplified by the use of link budgets

Evaluation the received power and noise power in radio link

The link budget must be calculated for individual transponder and for each link

When a bent pipe transponder is used the uplink and down link C/N rations must be combined to give an overall C/N

3 June 2008

Satellite Link Design – Satellite Link Design – Downlink received PowerDownlink received Power

The calculation of carrier to noise ratio in a satellite link is The calculation of carrier to noise ratio in a satellite link is based on equations for received signal power Pbased on equations for received signal power Pr r and and

receiver noise power:receiver noise power:

PPrr = EIRP + G = EIRP + Grr – L – Lpp – L – Laa – L – Ltata – L – Lrara dBW, dBW,

Where:Where:

EIRPEIRP = 10log= 10log1o1o (P (PttGGtt))

GGrr = 10log= 10log1010 (4 (4AAe e / / 22)dB)dB

PathLoss LPathLoss LPP = 10log= 10log10 10 [(4 [(4 AAe e / / ) 2] = 20log) 2] = 20log1o1o (4 (4R/ R/ )dB)dB

LLaa = Attenuation in the atmosphere= Attenuation in the atmosphere

LLtata = Losses assosiated with transmitting antenna= Losses assosiated with transmitting antenna

LLrara = Losses associated with receiving antenna= Losses associated with receiving antenna

3 June 2008

Satellite Link Design – Satellite Link Design – Downlink Noise PowerDownlink Noise Power

A receiving terminal with a system noise temperature Ts K and a noise bandwidth B Hz has a noise power Pn referred to the output terminals of the antenna where

Pn = kTsB watts

The receiving system noise power is usually written in decibel units as:

N = k + Ts + B dBW,

where:

k is Boltzmann’s constant (-228.6 Dbw/K/Hz)

Ts is the sytem noise temperature in DBK

B is the noise Bandwidth of the receiver in dBHz

3 June 2008

Satellite Link Design – UplinkSatellite Link Design – Uplink Uplink design is easier than the down link in many cases

– Earth station could use higher power tranmitters

Earth station transmitter power is set by the power level required at the input to the transporter, either– A specific flux density is required at the satellite – A specific power level is required at the input to the transporter

Analysis of the uplink requires calculation of the power level at the input to the transponder so that uplink C/N ratio can be found

With small-diameter earth stations, a higher power earth station transmitter is required to achieve a similar satellite EIRP. – Interference to other satellites rises due to wider beam of small

antenna

Uplink power control can be used to against uplink rain attenuation

3 June 2008

Propagation Effects & their ImpactPropagation Effects & their Impact Many phenomena causes lead signal loss on through the earths

atmosphere:– Atmospheric Absorption (gaseous effects)– Cloud Attenuation (aerosolic and ice particles– Tropospheric Scintillation (refractive effects)– Faraday Rotation (an ionospheric effect)– Ionospheric Scintillation (a second ionospheric effect)– Rain attenuation– Rain and Ice Crystal Depolarization

The rain attenuation is the most important for frequencies above 10 GHz– Rain models are used to estimate the amount of degradation (or

fading) of the signal when passing through rain. – Rain attenuation models: Crane 1982 & 1985; CCIR 1983; ITU-R

p,618-5(7&8)

3 June 2008

Propagation Effects and their Propagation Effects and their Impact IIImpact II

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